1. Field of the Invention
The present invention relates generally to a system for controlling the emission of dust generated during the loading and unloading of an open-faced cargo hold. More particularly, the invention pertains to a hatch covering that prevents the escape of dust from therebelow while permitting ready access to the cargo by cargo handling equipment.
2. Description of the Prior Art
Due to increasing concern regarding the pollution of the environment, increasingly stringent restrictions are being imposed on various industries in an effort to reduce the amount of dust released into the atmosphere. While simply placing a covering over a source of dust would theoretically eliminate a dust problem, substantial practical impediments are encountered when dealing with large areas and when the area below the cover must be freely accessible.
Such problems are especially pronounced in the loading and unloading operation of bulk cargo as for example the handling of a cargo of cement in the hold of a freighter or barge. An opened cargo hold exposes an expansive area, loading and unloading equipment must be able to reach the far corners of the hold's interior without hinderance, while the handling of cement generates huge amounts of fine dust.
The unloading of bulk material such as cement is typically achieved with a device such as is described in Johansson, et al., U.S. Pat. No. 4,793,467. The device generally consists of a vertical screw conveyor housed within an elongated tubular structure that is inserted into the bulk material in order to draw the material upwardly and on to a horizontal conveyor. The intake end of the device must make direct contact with the bulk material and consequently, in order to be able to reach the peripheries of a typical hold configuration through a hatch opening of relatively reduced dimensions, the screw conveyor is manipulatable through a wide range of angles and depths. While a bulk materials loading device does not need the full freedom of movement required of an unloading device, sufficient mobility must nonetheless be available in order to achieve an even distribution within the hold. A substantial amount of dust is raised during the operation of such devices.
Hatch covers that prevent the raised dust from escaping to the atmosphere, must be able to accommodate the shifting and angling of the loading and unloading equipment. Cover systems that have been developed in the past suffer from substantial shortcomings in this respect. The system described in Swedish Patent No. 8900438-6 consists of a tarpaulin-type covering having an array of access apertures therein. Each aperture has its own removable tarpaulin type cover. Whenever the cargo handling equipment must be relocated from one aperture to another, a cover must be removed manually, the conveyor withdrawn from the hold and reinserted via the newly opened aperture while the previously used aperture must be manually covered and its cover secured. A fairly labor intensive effort is thereover required in order to take advantage of this system.
Alternatively, applicants have previously considered a substantially more complex system in which a first rigid frame is suspended across an open hold so as to enable its position to be longitudinally shifted along the hold. A second smaller rigid frame cooperates with the first frame so as to allow its position to be shifted laterally relatively thereto. By coordinating the movements of the two frames, complete access to the hold below can be achieved. Two sections of tarpaulin type material are suspended across the hold fore and aft of the first frame so as to provide coverage. Each section is of sufficient area to provide adequate coverage on one side of the frame regardless of the actual position of the first frame. The tarpaulin is suspended in a manner so as to allow excess material to accordion as the first frame is shifted towards the for or aft side of the hold. A similar tarpaulin configuration is provided on either side of the second frame within the first frame.
Although such a system theoretically provides good access to the hold while preventing the escape of dust, the system suffers from substantial shortcomings. Hatch dimensions do not conform to a universal standard, and it may be difficult if not impossible to adapt the described dual rigid frame system to undersized or oversized hatch configurations. The mechanical complexity inherent in such a system, wherein a plurality of wheeled suspension systems acting in concert with one another and wherein large number of fittings are necessary in order to attach the various tarpaulins so as to enable an accordioning action thereof when called for renders such a system prone to failure. Moreover, should a failure occur that results in either a restriction of movement of the frames or an excessive escape of dust, the entire loading or unloading operation must cease until a repair is completed.
A dust control system is therefore called for that is adaptable to a wide variety of hatch configurations; once installed, requires only a minimal amount of manual manipulation during the cargo handling operation; is not easily damaged, although if damaged, allows cargo handling operations to continue while repairs are being performed; and, is relatively inexpensive to manufacture.